I, too, had always assumed "Hogwarts" was a spoonerism of warthog, given the winged boar statues on either side of the school gates, but I just searched Accio Quote and found that JKR revealed in an interview back in 2001 that the school's name was unconsciously inspired by a plant called hogwort ("-wort" being a common suffix in plant names):

"Ideas come from all sorts of places and sometimes I don't realise where I got them from. A friend from London recently asked me if I remembered when we first saw Hogwarts. I had no idea what she was talking about until she recalled the day we went to Kew Gardens and saw those lilies that were called Hogwarts. I'd seen them seven years before and they'd bubbled around in my memory. When Hogwarts occurred to me as a name for the school, I had no idea where it came from."